Drills for hole making in workpieces of metal by chip removing or cutting machining may be divided into different categories depending on a number of different factors, such as the desired shape, length and diameter of the holes that are to be made, the nature of the materials in the workpieces, the desired dimensional accuracy in the holes, the fundamental construction or design of the drill, etc. Among other things there is a sharp borderline between, on one hand, solid drills and, on the other hand, non-solid drills, the first-mentioned ones of which are distinguished by the fact that all the necessary details, such as cutting edges (with the ensuing chip surface and clearance surface), chip evacuation channels, guide pads, flushing fluid channels, etc., are included in a solid drill body of a suitable material, such as cemented carbide, high speed steel, or the like. Refer to, for instance, p. E32 in the catalogue “Cutting Machining Technical Guide,” published by AB Sandvik Coromant October 2005. A fundamental advantage of solid drills is that all details, and in particular the cutting edges, can be manufactured with high precision and dimensional accuracy. They are therefore suitable for applications in which the requirements of the position precision and the dimensional accuracy of the holes are high. They are used to a great extent also for the drilling of holes of a limited diameter (e.g. <12 mm). A drawback with solid drills is, however, that they have to be either discarded or, if possible, reground when the cutting edges have been worn out. This may at times cause expenses that may be high.
The category of non-solid drills is heterogeneous and includes a plurality of different types of drills, such as indexable insert drills, loose top drills and drills having soldered cutting inserts. Common to these types of drills is that the cutting edges required to carry out chip removal are included in particular wear parts, which are manufactured from another, harder and more hard-wearing material than the material otherwise in the drill body. In indexable insert drills, the cutting edges are included in replaceable, indexable inserts, and in loose top drills, the same are included as integrated parts of a replaceable loose top, which can be interconnected with a reusable drill or basic body. Also in the third type of drills, the cutting edges are included in cutting inserts that, like the indexable inserts, usually consist of cemented carbide (or alternatively useful, hard materials), but that are soldered or otherwise semi-permanently united to the proper basic body (that usually consists of steel). An advantage of non-solid drills is that the same do not necessarily have to be discarded or require regrinding of the cutting edges, when their sharpness has been lost. Accordingly, indexable inserts and loose tops can rapidly and easily be exchanged in a mechanical way, while soldered cutting inserts can be removed in a thermal way and either be reground or replaced by new ones. In other words, the proper basic body or drill body is in this case reusable for a plurality of insert exchanges (usually 10-20). A disadvantage of all types of non-solid drills is, however, that the drill body and the cutting edge-carrying wear parts are not integrated with each other, but rather of mutually different nature, above all in respect of the properties of the different materials (e.g., steel/cemented carbide), besides which the wear parts have to be connected or interconnected with the basic body via interfaces in which sources of error may arise. In other words, non-solid drills are less reliable when high precision is desired.
Another borderline between two main categories of drills goes between single drills and step drills. Single drills include only one set of cutting edges in a front tip of the drill body and can make a hole having one and the same diameter along the entire length thereof. The step drill, however, can in one and the same operation drill holes in consecutive sections having different diameters, more precisely by including not only one set of cutting edges in the tip of the drill body, but also in one or more so-called steps that are formed at a distance behind the tip and have a greater diameter than the tip, and include additional cutting edges that can generate additional hole sections having a successively increasing diameter. To the category of step drills belong, furthermore, a number of varying types of drills, such as twist drills (having helicoidal chip flutes) and tap borers (having straight chip flutes). The step drills may furthermore be formed either as solid drills or as non-solid drills, e.g., indexable insert drills or drills having soldered cutting inserts.
An important factor in order for drills most generally to work satisfactory and give good machining results is that the chip formation and the chip evacuation are carried out in a way that is expedient for the application in question. In certain cases, drills are used for hole making in short-chipping materials, i.e., materials having a low ductility, such as cast iron, wherein the removed chips become fairly short (often comma-shaped) and easy to evacuate via the chip flutes of the drill. However, other, more ductile materials, such as low-carbon steel, aluminium, copper, titanium and acid-proof steel, generate most often long chips which are difficult to handle, which may cause the most detrimental problems such as chip jamming, impaired hole quality, entanglement in the driving machine, and even risks of accidents. In step drills, these problems may become accentuated as a consequence of the fact that at least two types or sets of long chips are to be evacuated via mutual chip flutes, viz. not only a first set generated by the primary cutting edges in the tip of the drill, but also at least one second set generated by the secondary cutting edges in the step or steps of the drill. Because all chips have to be evacuated via common chip flutes, the risk of entanglement and chip jamming becomes extra great.
In this connection, it should also be pointed out that narrow chips generated by short cutting edges often remain long and unbroken, while wider chips generated by longer cutting edges are inclined to break easier and become acceptably short. Another factor that affects the chip formation is the angle between cutting edges co-operating in pairs. Hence, the primary cutting edges usually have a so-called nose angle within the range of 120-160°, while the corresponding angle (step angle) between the secondary cutting edges may vary most considerably all the way from 180° to approximately 40°. If the step angle is great, the chip is directed more axially than if the same is small. In the last-mentioned case, the chip is directed more radially and therefore gets an increased tendency to coil into tangles that may cause severe chip jamming.
The problems of mastering the forming and evacuation of chips in long-chipping materials have, at least to a certain extent, been solved in connection with non-solid step drills, viz. by the fact that already in connection with the manufacture of the separate cutting inserts applied to the drill afterwards, the cutting inserts are designed with special chip breakers, which at an early stage of the cutting process can break the chip and split the same into short pieces. An example of a step drill having semi-permanently attached cutting inserts is disclosed in WO00/44518, which shows how a laminated cutting insert having a built-in chip breaker can be soldered or otherwise semi-permanently attached to a step drill with the purpose of mastering the chip formation and evacuation. A disadvantage of non-solid drills in general and, in particular, of step drills is, however, that various sources of error may occur in connection with the mounting or fastening of the separate cutting inserts, more precisely in respect of the position precision of the cutting inserts and cutting edges in relation to the rest of the drill body. For this reason, non-solid step drills are poorly suitable for such hole making where the requirements of the position precision of the holes are particularly great.
The present invention aims at obviating the disadvantages of previously known solid step drills and at providing an improved solid step drill. An object of the invention is to provide a solid step drill that generates holes with high precision without giving rise to difficulties with the forming and evacuation of long chips. In doing so, not only the cylindrical hole walls made with different diameters and generated by the respective cutting edges should be possible to be given a high dimensional accuracy, but also the ring-shaped (usually conical) transition surface that is formed between the respective hole walls should be possible to be given a high dimensional accuracy.